The invention concerns long-haul optical links and in particular those that transmit a plurality of wavelength-division multiplexed optical signals. To compensate the losses caused by the distance, a link of the above kind is divided into a plurality of sections and an optical amplifier is inserted between two successive sections to compensate the losses. The transmission of a plurality of wavelength-division multiplexed optical signals necessitates the use of amplifiers having a relatively wide and flat passband to offer the same gain to all the optical signals, regardless of the carrier wavelength.
In the case of a doped fiber amplifier, the variations of the gain as a function of the wavelength, for a given length of fiber, are minimal for a certain value of its mean gain. To minimize the variations of gain as a function of wavelength, in order to transmit the greatest possible number of wavelength-division multiplexed optical signals, it is necessary to choose the mean gain value corresponding to the maximal passband for a fixed gain excursion. This optimal value of the mean gain depends only on the length of the doped fiber and the transmission losses of the amplifier components for a given type of fiber. The pump power is therefore chosen so that the mean gain has this optimal value. The value of the mean gain must remain constant for the passband to remain optimized. Amplifiers are therefore used in which the mean gain is regulated individually by a control system. A long-haul link is divided into sections the length of which gives rise to nominal losses such that they are exactly compensated by an amplifier optimized in this way.
On a long-haul link the losses can vary, i.e. increase over their nominal value, accidentally, even if the amplifiers remain perfectly operational, and with an individual gain that is perfectly stabilized. These variations in the losses can be due to variations in the characteristics of the optical fiber sections or of the connections between the optical fiber sections. It is then desirable to have at least one variable gain optical amplifier to compensate the variations in the losses on the link. However, it is not possible to vary the gain of a conventional optical fiber amplifier and at the same time retain the optimization of the response curve in the passband.
One solution to this problem is to associate a fixed gain amplifier, overrated compared to the nominal losses, with an optical attenuator on its output side. The amplifier then includes a doped fiber having a length greater than the length corresponding to the optimal gain and an optical pump the power of which is also greater than the power corresponding to the optimal gain. Because the power of a pump laser diode cannot easily be increased by more than 3 dB relative to the usual values, the maximal variation in the losses that can be compensated, expressed in decibels, is also limited to 3 dB.
Semiconductor optical amplifiers can also be used to compensate the losses on a long-haul link. The mean gain of an amplifier of this kind is regulated so that it does not fluctuate with the transmitted optical signals. It is generally regulated by operating the amplifier under laser conditions. This regulation process does not allow the mean gain to be varied to compensate an increase in the losses on the link.
Another solution is described in: SELF-REGULATING WDM AMPLIFIER MODULE FOR SCALABLE LIGHTWAVE NETWORKS, Goldstein et al, in OPTICAL AMPLIFIERS AND THEIR APPLICATIONS, TECHNICAL DIGEST, Series Volume 14, Aug. 3-5, 1994, BRECKENRIDGE Colo. This document describes an amplifier for a wavelength-division multiplex enabling each of the carriers to be amplified separately and thus eliminating the problem of maintaining a wide passband having a flat response curve. To amplify an optical multiplex including m carriers it includes:
an optical demultiplexer having m outputs and one input, PA1 an optical multiplexer having m inputs and one output, and PA1 m optical amplifiers having a common optical pump and each incorporating an independent doped fiber. PA1 m optical amplifiers each having a fixed gain different from the gains of the other amplifiers and a gain excursion as a function of wavelength at most equal to a common value in the given band; PA1 first switching means having an input coupled to the input of the variable gain amplifier, m outputs respectively coupled to the inputs of the m fixed gain amplifiers and a control input receiving the gain control signal; and PA1 second optical switching means having m inputs respectively coupled to the outputs of the m fixed gain amplifiers, an output coupled to the output of the variable gain amplifier and a control input receiving the gain control signal. PA1 a star coupler coupling the input of the variable gain amplifier to inputs of all the fixed gain amplifier simultaneously; and PA1 means for supplying power to one only of the fixed gain amplifiers, this amplifier being selected in accordance with the gain control signal.
The output of the demultiplexer corresponding to a given wavelength is connected by an independent amplifier to the input of the multiplexer corresponding to the same given wavelength. The m amplifiers have the same gain for each of the respective m wavelengths so that all the carriers are amplified the same. To vary the common gain of these amplifiers it is possible to operate on the power of the optical pump common to the amplifiers without altering the passband of the system. The variations in the individual passband of each amplifier are of little importance, since each amplifier has to amplify only one carrier. This prior art device has the disadvantage of requiring a number of optical amplifiers, a number of multiplexer inputs and a number of demultiplexer outputs that increase with the number m of optical carriers constituting the optical multiplex to be amplified. Also, the multiplexer and the demultiplexer cause high optical losses.